Different kinds of particulate medium-containing devices have been used for treatment of wastewater. Some types are intended primarily for chemical purification. Others are intended principally for physical straining of suspended solids.
For instance, in a typical sand filter, water is distributed continuously or intermittently on the top of a bed of sand having a depth of two feet or more. Sometimes, the water flows through the sand and directly into the underlying soil. Sometimes, drains at the bottom of the filter collect the water, so it can be flowed elsewhere. A filter may be created on or within the natural earth or it may be contained within a concrete, metal or plastic container. Shallow-bed sand filters of various types are used in combination with septic tanks for domestic and commercial wastewater.
Common silica sand or anthracite coal sand is commonly used as a medium. Typically the medium will have a maximum particle size of less than about 5 mm. Although water is often clarified upstream of sand filters to separate out entrained matter, there is still an undesirable tendency for organic matter to accumulate, especially near the top of the bed. The interstices amongst the grains of sand can be become largely blocked to passage of water, so that filter function is greatly reduced. Thus, to restore function, the bed may be backwashed, alternatively, the top surface of the bed is raked or replaced, and other times, all medium in the bed may be replaced.
A trickling filter is another kind of device, which is used for various purposes, including purifying wastewater, by action of microorganisms. Generally, a trickling filter is characterized by large particles and large interstices, compared to sand, with more resultant space within the medium. During use, wastewater is distributed on the top of the filter, so it passes over the medium and is subjected to action of microorganisms which populate the top surface of the medium, in what is often called a biofilm. Trickling filters may be operated in different ways with different oxygen concentrations. The microorganisms are most often predominantly aerobic. In special instances, predominately anaerobic, or facultative, microorganisms may be present.
A diversity of coarse mediums have been used for trickling filters. For instance, a manufactured medium may be comprised of molded or fractured ceramic pieces, or plastic objects. Often times the medium is hollow to create more surface area. Other mediums comprise wood chips, peat, stones, pieces of rigid or flexible plastic foams, strips, natural and synthetic fibers and mats, etc.
By its nature, a trickling bed filter will not filter suspended solids, even though some may incidentally adhere to the medium surfaces. From time to time, the biofilm sloughs off. Thus, some sort of settling chamber or pond, or a clarifier, is commonly positioned upstream and downstream of a trickling filter.
Air is often allowed, or forced, to circulate through a trickling bed filter. Depending on the spacing of the medium and the configuration of the vessel, there will be natural air circulation through the trickling bed filter. For example, see U.S. Pat. No. 3,542,937 to Choun and U.S. Pat. No. 2,293,174 to Robjohns. Forced airflow has been employed. See, for example, U.S. Pat. No. 2,308,866 to Dekema, which shows how air is flowed upwardly, counter to the down-flow direction of the wastewater. U.S. Pat. No. 2,200,580 to Pruss et al. and U.S. Pat. No. 5,980,739 to Jowett describe how air is flowed downwardly, or concurrently with the flow of water. U.S. Pat. Nos. 3,542,937; 2,293,174, 2,308,866; 2,200,580 and 5,980,739 are each hereby incorporated by reference in their entirety.
Being coarse, and having large interstitial spaces, the medium in a trickling bed filter generally will not retain much moisture when drained. One problem with trickling filters is that if excessive air is flowed through the medium between doses of wastewater, the biofilm within the medium, especially the macropores, can be dried out, and that can may result in death of the microbes, resulting in adverse performance when a wastewater dose is subsequently applied.
There are various criteria by which the usefulness of prior art devices are measured. They include the cost of medium and the container; the efficacy of the system, measured by the quality or degree of treatment; the efficiency, insofar as the volume of wastewater satisfactorily processed per unit time for a given size unit, economic investment and operating cost; the durability or lifecycle of the unit and medium; and, the reliability and maintenance requirement, including cleaning and renovation of the medium. There is a continuing search for improvements in particulate-containing devices which process wastewater.